1. Field of the Invention
The present invention relates to a system for controlling chopping for a converter in a DC electrical power supply.
2. Description of the Prior Art
A DC power supply enables a DC voltage to be obtained either from an alternating electrical supply source or from a DC electrical supply. The circuit which enables this transformation to take place is termed a converter. The converter needs to be designed in such a way that the power supply causes the output voltages to be galvanically isolated from the main supply, and supplies one or more DC voltages which exhibit very low residual ripple. These voltages need to be highly stable no matter what changes may take place in current drain, the source supply voltage or ambient temperature.
The construction of a power supply depends upon the size of the machine to which power is supplied. In the field of high-capacity power supplies, the current trend is towards the decentralization of functions. This has resulted in the appearance of genuine power-supply "systems" in which various kinds of converter sub-assemblies can be distinguished. For example, there are
(1) AC to DC converters which produce a DC voltage from an alternating supply source.
(2) AC to AC converters which produce AC voltage of higher frequency than the frequency of an alternating supply source.
(3) DC to AC converters which produce an AC voltage from a DC supply source and which are generally referred to as inverters.
(4) DC to DC converters which produce a DC voltage from a DC supply source. These are chopping converters. In this kind of converter the DC supply output voltage is chopped by a signal of more or less high frequency.
Converters of the DC to DC kind may be further divided into two categories. The first type are known as asymmetrical converters in which the magnetic core of the isolating transformer (if there is one) is excited in only one direction. The second type are known as symmetrical converters in which the magnetic core of the isolating transformer is excited alternately in one direction and then in the other. A drawback of these converters is that they require conduction-restricting limiters to avoid damage to the chopping transistors connected to the chopping transformer.
A principal object of the present invention is to provide an improved chopping control system for controlling the chopping transistor of a power supply which makes use of a symmetrical converter. The improved system of the invention makes it possible to reduce the switching time of the chopping transistor while preventing its rapid destruction. It also enables the residual ripple level of the power supply to be reduced, while at the same time providing good galvanic isolation between the input and output of the power supply.